Transfer of ions across biological membranes is central to physiological processes like nerve excitation, muscle cell contraction, signal transduction and hormone secretion. Ion channels play a vital role by providing a passageway, the ion conduction pore, within membranes to allow specific ions to traverse down their electrochemical gradient. In humans, ion channels are found in nearly all tissues serving a variety of physiologically essential functions. Because of their prevalence and importance in the human body, dysfunction of a channel is often to blame for a wide range of human pathologies. The ability to select for specific ionic species is known as ion selectivity and is a fundamental property defining ion channel function. In tetrameric cation channels, which comprise the single largest family of ion channels including the K+, Ca2+, Na+, and cyclic nucleotide-gated (CNG) channels, selectivity is usually a direct consequence of the unique structural and chemical environment within part of the ion channel pore, the selectivity filter, which is distinct among different channels. Our understanding of the molecular details governing ion selectivity in this group of channels has come a long way with the advancement of genetic, biochemical, and electrophysiological analysis of ion channels and, more recently, the structural characterization of several members beginning with the ground breaking work of the KcsA K+ channel structure and subsequent work on other K+ selective channels. Although these structural studies offer a direct visualization of the selectivity filter of K+ channels, the determinant factors contributingto K+ selectivity are still under heated debate. Moreover, there is little structural information available for other tetrameric cation channels and the molecular basis for their ion selectivity remains unclear. The overall goal of our research is to understand the structural basis of ion selectivity in tetrameric cation channels. Taking advantage of the extremely high resolution crystal structures of several model proteins representing the ion conduction pores of both selective and non-selective channels, we aim to elucidate basic principles of ion selectivity in two groups of physiologically essential cation channels. One is the non-selective, Ca2+ permeable cyclic nucleotide-gated (CNG) channels, using NaK from Bacillus cereus and its CNG- mimicking chimeras as model systems; the other is K+ selective channels, using a K+ selective NaK mutant (NaK2K) and the MthK K+ channel from Methanobacterium thermoautotrophicum as model systems. A combined approach of protein crystallography, electrophysiology and protein chemical synthesis will be employed in the proposed studies